1. Field of the Invention
The present invention relates to an electrostatic image forming apparatus capable of reducing defective discharge and defective image transfer ascribable to the deposition of free toner particles on a corona discharger.
2. Discussion of the Background
In a conventional electrostatic image forming apparatus, a corona discharger is located so as to face an image carrier on which a toner image is to be formed. When a paper or similar recording medium is brought between the image carrier and the discharger, the discharger effects corona discharge at a rear end of the paper so as to charge the paper to a polarity opposite to a polarity of toner forming a toner image. As a result, the toner image is transferred from the image carrier to the paper. This type of apparatus will be referred to as Prior Art 1.
In another conventional electrostatic image forming apparatus, charging means, exposing means, developing means and primary transfer means are arranged around a photoconductive element while an intermediate transfer medium is interposed between the photoconductive element and the primary transfer means. Secondary transfer means is positioned around the intermediate transfer medium. A paper or similar recording medium is passed through a gap between the intermediate transfer medium and the secondary transfer means. In operation, the charging means uniformly charges the surface of the photoconductive element to a preselected polarity. The exposing means exposes the charged surface of the element in accordance with image data, thereby forming a latent image. The developing means develops the latent image with toner to thereby produce a corresponding toner image. The primary transfer means is applied with a voltage opposite in polarity to the toner image so as to transfer the toner image to the intermediate transfer medium. The secondary transfer means is applied with a voltage opposite in polarity to the toner image carried on the intermediate transfer medium so as to transfer the toner image to the paper. This type of apparatus will be referred to as Prior Art 2.
To form a full-color image, Prior Art 1 sequentially forms color images of different colors on the image carrier one above the other, and then collectively transfers the resulting laminate or composite image to the paper. On the other hand, Prior Art 2 repeats the primary transfer to form the laminate toner image on the intermediate transfer medium, and then transfers it to the paper.
The intermediate transfer medium is implemented as a belt or a drum having medium resistance, i.e., a volume resistivity of 1.times.10.sup.8 .OMEGA.cm to 10.sup.12 .OMEGA.cm and a surface resistivity of 1.times.10.sup.8 .OMEGA. to 10.sup.11 .OMEGA., as measured by a test prescribed by JIS (Japanese Industrial Standards) K6911. Should the transfer medium have high resistance, its charge potential would sequentially increase due to a bias repeatedly applied thereto for the primary transfer, rendering the primary and secondary transfer defective. If the intermediate medium has medium resistance and discharges by way of a conductive support member supporting it, then the charge of the medium can be maintained substantially constant despite the repeated primary transfer during the formation of a full-color image.
For the transfer of the toner image, use has customarily been made of a bias roller or a corona discharger. As for a bias roller, if it contacts the image transferred by the primary transfer means during the formation of a full-color image, then it disturbs the image. Hence, the bias roller must be spaced from the intermediate transfer medium until the image transferred by the primary transfer means moves away from the roller. This cannot be done without resorting to a mechanism for moving the bias roller into and out of contact with the intermediate transfer medium. Such a mechanism is complicated and bulky resulting in extra cost.
In light of the above, the bias roller may be replaced with a corona discharger which does not contact the toner image at all, as proposed in relation to both of Prior Art 1 and Prior Art 2.
In Prior Art 1 and Prior Art 2, discharging means in the form of a brush or needles may be used to separate the paper moved away from an image transfer position from the image carrier by discharging the paper charged by the corona discharger, as needed. In the event of image transfer, an AC voltage or a voltage opposite in polarity to the voltage applied to the corona discharger is applied to the discharging means. Alternatively, the discharging means may be simply connected to ground without any voltage applied thereto. This type of apparatus will be referred to as Prior Art 3.
The apparatus using the intermediate transfer medium having medium resistance has the following problems left unsolved. The adhesion of the toner image to the intermediate transfer medium is weaker than the adhesion of the same to the image carrier, so that the toner is easily isolated from the intermediate transfer medium. Moreover, in the event of image transfer to the paper, the intermediate transfer medium passes by the secondary transfer means three consecutive times. When the color switching time available for the developing means is short, the medium passes by the secondary transfer means even six consecutive times because a single idle rotation occurs between the consecutive transfers.
In any case, when the toner image is conveyed via the secondary transfer means by the intermediate transfer medium, isolated or free toner particles appear due to the weak adhesion of the toner image to the intermediate transfer medium. This is particularly true with the laminate toner image portion. When discharge begins for the secondary transfer, the free particles are electrostatically attracted by the discharge wire of the corona discharger. The free particles deposited on the discharge wire results in defective discharge and defective image transfer. This occurs not only in Prior Art 2 but also in Prior Art 1.
In Prior Art 3, the discharging means, like the corona discharger, is applied to a color image forming apparatus using the intermediate transfer medium. In this case, the toner deposited on the intermediate transfer medium flies about and contaminates the discharging means when the toner image passes through the image transfer position. It is likely that the toner deposited on the discharging means is transferred to the rear of the paper. To solve this problem, a voltage of the same polarity as the toner may be applied to the discharging means. However, the discharging means is different from the wire and casing of the corona discharger in that it is positioned extremely close to the surface of the image carrier. In addition, the discharging means, whether it be a brush or needles, is apt to cause discharge to occur between its tips and the surface of the image carrier. The discharge electrically disturbs the toner image carried on the image carrier. Although the discharging means may be simply connected to ground without any voltage applied thereto, this kind of scheme cannot prevent the toner from flying away from the image carrier and therefore fails to free the discharging means from contamination.